The direct reduction of iron ore to the product sponge iron in the form of DRI (Direct Reduced Iron) or HBI (Hot Briquetted Iron) is undertaken using heated process gases, preferably in a shaft furnace. In simple terms this involves chemical reactions which convert iron ore (Fe203) and natural gas (CH4) into iron Fe, carbon dioxide CO2 and water H20.
The shaft furnace is in continuous operation in that raw material is constantly fed into it from above in the form of iron ore pellets and the sponge iron is withdrawn at the bottom in a similar continuous process.
Methods of producing sponge iron are for example known from U.S. Pat. No. 4,093,455, U.S. Pat. No. 4,178,151, U.S. Pat. No. 4,234,169 and WO 02/097138 A1. Further publications dealing with this subject are: Thompson, M.: “Control Innovations in MIDREX Plants: An Introduction” in “Direct from MIDREX”, 1st Quarter 2001, P. 3-4, 2001 and Görner, F., Bacon, F.: “Development of Process Automation for the MIDREX Process” in “Direct from MIDREX”, 1st Quarter 2002, P. 3-5, 2002.
In the production of sponge iron it is desirable to produce a product with properties which are as constant as possible and precisely specified. To this end it is known that all factors which might influence the product to be produced are to be kept as constant as possible and thereby the process is to be operated at a known working point. However even the assumption of a fully homogeneous iron ore as raw material for example is often not fulfilled.